a. Field of the Invention
This invention relates to a method and apparatus for the cathodic protection of a structure such as a pipeline, well casing etc. and more particularly to a method and apparatus for providing a pulsed d.c. voltage and current to the structure.
b. Description of the Prior Art
The use of cathodic protection to prevent corrosion is well established for the protection of metal structures, such as well casings and pipe lines, that are buried in conductive soils. Cathodic protection is also used for the protection of inner surfaces of tanks which contain corrosive solutions, as well as for the protection of subplatforms, and other offshore metal structures. It is well established that the cathodic protection can be accomplished either by the use of sacrificial anodes electrically grounded to the structure to be protected, or by the application of low voltage direct current from a power source. In the latter method steady direct current, half or full wave rectified current, and pulsed direct current have all been used.
It has been well established that, when a cathodic protection current is applied to a circuit including the structure (cathode) to be protected and its associated anode, a layer of charge is formed at approximately 100 A. from the surface of the structure. This layer of charge is called a taffel double layer. This layer acts as a capacitor in series with the anode-cathode circuit.
The structure to be protected, such as a pipeline or well casing, the anode and the leads connecting such elements to the voltage source act as an inductive (as well as a resistive) load to the current flow. The soil between the anode and the structure also provides a resistive load of less than one to several ohms.
In the absence of a cathodic protection system the soil or other conductive corrosive medium to which a ferrous metal structure such as a steel pipeline is exposed will cause an adverse chemical reaction in which ferrous or iron molecules pass into solution as positive ions by surrendering electrons to the structure. Hydrogen ions in the solution will accept the free electrons and form a gas e.g. H.sub.2 adjacent to the surface of the structure. Oxygen molecules and certain other substances, if present in the solution, will also accept the electrons. This action results in a loss of iron in the structure with a consequent degradation of structural integrity.
Direct current cathodic protection systems prevent (or inhibit) the iron molecules from passing into solution by providing an exterior source of free electrons to the structure. The electrons supplied by the cathodic protection systems reduce any oxygen molecules and/or hydrogen ions present at the surface of the structure. The iron molecules are inhibited from going into solution, because the hydrogen ion and oxygen molecule receptors for the iron molecule electrons have been reduced by the cathodic protection system electrons. As a general rule, the greater the amount of current (accumulated electrons per unit of time) that is supplied by the cathodic protection system, the greater will be the area of structure protected.
A typical steady state 15 volt and 15 ampere d.c. cathodic protection system offers good protection but provides only a limited umbrella of protection or throw along the structure such as a pipeline to be protected. Such steady state systems thus require a considerable number of protection stations for a given length of the structure or pipe to be protected. Increasing the amount of current supplied by increasing the voltage, will increase the throw. The average current must, however, be limited such that an excess of hydrogen gas is not generated at the point of application of the cathodic protection system. An excess of hydrogen may cause damage to protective coatings. Excess hydrogen will also permeate the pipe wall, causing certain pipe materials to crack or rupture.
It has been shown that a pulsed d.c. voltage source having an output of the order of 100-300 volts for 5-100 microseconds (".mu.s") with a duty cycle of the order of 10% provides a much greater coverage (or throw) per station e.g. one station every few miles of pipeline. Such pulsed systems have been considered to be particularly effective because, although the average current is still in the order of magnitude of 15 amperes, the peak current, which is flowing for a sufficient length of time to cause the protective reactions to take place, will be typically as high as 300 amperes. The pulsed d.c. systems also cause a greater redistribution of the current along the structure, such as a pipeline, because of the inductive and capacitive reactance of the anode and structure system.
A major problem which occurs in the prior art cathodic protection systems is the stray current interference of the systems when two or more structures are located adjacent or near each other. This problem is best illustrated in FIG. 1 of the drawings where reference numeral 10 designates a pulsed d.c. source such as those described in U.S. Pat. Nos. 3,612,898 and 3,692,650 of which I am named as a co-inventor. The d.c. source is connected across a positive terminal 12 and a negative terminal 14 which terminals are in turn connected by appropriate leads to an anode device 16 and the structure to be protected such as a pipeline 18 which acts as the cathode. The anode device generally consists of several discrete metal cylinders connected in parallel and spaced from each other in one or more holes extending several hundred feet below ground level. A diode 20 is connected across the positive and negative terminals to allow the current induced by the emf resulting from inductive reactance of the anode-cathode load at the end of the voltage pulse to pass freely from the negative to the positive terminal. This arrangement prevents the negative terminal 14 from going positive with respect to the terminal 16 (except for the very small diode breakdown voltage) and thus protects the voltage source from a reverse voltage spike. However, the arrangement allows current (represented by waveform I.sub.1 in FIG. 1) to continue to flow in the load for a considerable time after the termination of the voltage pulse (represented by waveform V.sub.1 in FIG. 1).
Pulsed current flowing in the anode/cathode circuit or load, although less than with steady state systems, may adversely affect neighboring ferrous metal or steel structures (e.g. the pipeline 22 of FIG. 1) which intersect the anode electric field and pass near the protected structure. For example, current will flow from an area 23 of the pipeline 22 to a point 24 located opposite (and nearest) the protected pipeline 18. At point 24 iron molecules will surrender electrons to the pipe 22 to satisfy the current demand and go into solution. As a result a hole will be formed at point 24 taking the pipeline 22 out of service until an appropriate repair is made.
A sacrificial anode may be placed on the pipeline 22 near the point 24 or the two pipelines may be connected by a conductive wire to prevent the perforation of the metal. However, sacrificial anodes must be replaced and a wire connection between the structures will reduce the area of protection for pipeline 18 (and perhaps pipeline 22) and create additional problems in the event that the protection system for either pipeline is inactivated. The liability problems resulting from damage to neighboring pipelines can be very significant.
There is a need to reduce or eliminate the current flow due to the inductive reactance in a pulsed d.c. cathodic protection systems to thereby minimize any adverse affects on neighboring ferrous metal structures.